Fixed polygonal edges with explicit test for point-on-line segment

Closes pull #136

src/main/java/com/sk89q/worldedit/regions/Polygonal2DRegion.java

@@ -302,6 +302,7 @@ public class Polygonal2DRegion implements Region {
         int xOld, zOld;
         int x1, z1;
         int x2, z2;
+        long crossproduct;
         int i;
 
         xOld = points.get(npoints - 1).getBlockX();
@@ -325,10 +326,14 @@ public class Polygonal2DRegion implements Region {
                 z1 = zNew;
                 z2 = zOld;
             }
-            if ((xNew < targetX) == (targetX <= xOld)
-                    && ((long) targetZ - (long) z1) * (long) (x2 - x1) <= ((long) z2 - (long) z1)
-                    * (long) (targetX - x1)) {
-                inside = !inside;
+            if (x1 <= targetX && targetX <= x2) {
+                crossproduct = ((long) targetZ - (long) z1) * (long) (x2 - x1)
+                    - ((long) z2 - (long) z1) * (long) (targetX - x1);
+                if (crossproduct == 0) {
+                    if ((z1 <= targetZ) == (targetZ <= z2)) return true; //on edge
+                } else if (crossproduct < 0 && (x1 != targetX)) {
+                    inside = !inside;
+                }
             }
             xOld = xNew;
             zOld = zNew;